The present invention has for its object to provide an equalizing circuit intended for producing an output current which has a given current characteristic as a function of the value of an input current the current characteristic having a maximum and a minimum value, which circuit comprises a means for producing a first current which is proportional to the input current, connected in series with a first current source producing a second current which is proportional to a reference current, in order to produce a first resulting current which is positive when the first current is higher than the second current and zero in the opposite case, a first switching stage arranged for being non-conductive when the first resulting current is zero, and passed through thereby when it is positive a second switching stage arranged for being passed through by the same current as the first switching stage, a second current source arranged for supplying a third current which is proportional to the reference current and connected in series with the second switching stage, in a manner such as to generate a second resulting current which is positive when the third current is higher than the resulting first current, a third switching stage arranged for not conducting when the second resulting current is zero, and passed through thereby when it is positive a first current output stage arranged for supplying a current which is equal to the current passing through the third switching stage and a second current output stage connected in parallel with the first current output stage arranged for supplying a current which is equal to the reference current, whose value is thus the said minimum value of the output current characteristic.
Such a circuit has been used for effecting a gain control of a telephone receiver, which circuit is commercially available referenced as TEA 1061.
A circuit having the same object but having a different structure has also been described in the U.S. Pat. No. 3,810,032.
The two circuits enable obtaining a current which linearly depends on an input current, between a bottom and a top limit. Such circuits are used specifically in subscriber telephone sets which are fed with a given voltage, generally 48 volts, from an exchange, the current which the subscriber terminal disposes of being smaller the more remote from the exchange the terminal is situated. In order to make the sensitivity of the telephone set independent of the length of the subscriber line and thus homogenize the listening level among the subscribers the subscriber set comprises an amplifier stage whose gain ratio depends on the current supplied and the above supply current is provided by an equalizing circuit fed by the line current, the equalization being effected according to a linear relationship as near to the theoretical curve as possible.
However, if the lines are very long and/or when moderate lengths of line sections are used, the theoretical curve stretches out over a larger dynamic range and tends to become more curved.
The above linear equalization is thus no longer sufficiently precise, which is unfavorable for certain subscriber stations.